1. Field of the Invention
The present invention relates generally to faceplates which are used in network switching apparatus and, more particularly, to extruded faceplates useful for protecting the network switch from electromagnetic interference (EMI) and electrostatic discharges (ESD).
2. Description of the Related Art
Extrusion is a process in which heated or unheated material, such as plastic, is pushed across a shaping orifice, known as a die, that continually shapes the material into a desired form. (See for example, Tadmor et al., "Principles of Polymer Processing, John Wiley & Sons, New York, pp. 3-7 (1997)). Conventional products formed by extrusion include wires, cables, rods, tubes, pipes and various profiles. For many applications, the desired attributes of the final part cannot be fully achieved with a single plastic material.
Coextrusion is used to combine two or more different materials in the form of a melt through the same die. In a multi-manifold die, the polymer melt streams are confined to individual flow channels until being joined before or directly at the primary die. (See, Encyclopedia of Polymer Science and Engineering, Vol. 6, pp. 608-613 (1988)). Coextrusion allows for multiple materials to be processed concurrently providing the use of, for example, a low-cost, thick structural internal material layer and a thin external material layer for aesthetics, abrasion resistance, weatherability and the like.
Conductive housings are used to provide shielding of a device from electromagnetic radiation. The disruption of an electronic device as a result of electromagnetic radiation is termed electromagnetic interference (EMI). EMI in the context of this disclosure typically includes frequencies within a range of about 50 hertz to about 10 gigahertz. Absorption and reflection energy losses provided by a shield material are proportional to: (1) the thickness of the shield, (2) the magnetic permeability of the shield material, (3) the frequency of the EMI wave, and (4) the surface resistivity of the shield material. While the defining relationships (i.e. frequencies) for refelction and absorption mechanisms are different, EMI shielding effectiveness is enhanced for materials which have a surface resistivity in the range of about 10.sup.-2 to 10 ohm/square. (For the purpose of this discussion, EMI shielding effectiveness will be described in terms of the surface resistivity, however, it is understood that the EMI shielding effectiveness is optionally described in terms of the volume resistivity of the shield material.)
U.S. Pat. No. 4,327,832 describes a container for packaging semiconductor components formed of a coextrusion of electrically conductive and electrically non-conductive plastics. The coextrusion has a body portion formed of electrically conductive opaque plastic to provide EMI shielding and a non-conductive transparent plastic to permit inspection of components within the container.
In addition to EMI shielding, it is desirable to protect electrostatically sensitive electronic components and devices from electrostatic discharge (ESD). The term electrostatic discharge as used in this disclosure refers to the discharge of static electricity from an essentially non-conductive surface. Enclosures are used for dissipating accumulated electric charge to ground. The enclosures typically have surface resistivities in the range of about 10.sup.5 to 10.sup.10 ohm/square. Materials which exhibit surface resistivities within the range are called static dissipative materials. Materials having surface resistivities within such range, slowly discharge the accumulated electric charge to the ground potential without generation of high current impulse (current spike). For example each current spike becomes spread in time, e.g., over 1/100 second for a surface resistivity of about 10.sup.10 ohms/square. U.S. Pat. No. 4,906,944 describes a package for an electrostatically sensitive component formed of a first layer of a coextruded film of polyolefin and a copolymer selected from ethylene-acrylic and ethylene-vinyl acetate copolymers, a second layer of an electrically conductive material laminated to the first layer and an outer layer of a static dissipative material adjacent to a side of the second layer opposite the first layer.
A typical conventional telecommunication transmission switch configuration includes a series of bays mounted side-by-side. Each bay has a number of shelves for parallel mounting of circuit packs. Circuit packs comprise electronic circuit boards and electronic components which perform electronic switching. A faceplate is attached to the front edge of each circuit pack. The circuit pack/faceplate assembly is held into the shelves by latches mounted to the top and bottom edges of the faceplates. It is desirable to provide EMI shielding and ESD immunity for the switch. ESD immunity is required to prevent electrostatic discharges, usually carried by craft personnel and their equipment, from discharging through the faceplates and latches onto the circuit packs.
Several types of conductive enclosures which shield network switches from EMI and ESD are known in the art. For example, a conductive enclosure made of metal doors is used to cover the front of each bay, thereby completely covering the faceplates. Also, conventional faceplates formed of metal have been used to provide EMI and ESD shielding. However, metal enclosures and faceplates can be costly, have inherent design limitations and typically require secondary processing, i.e., painting, to produce an acceptable aesthetic surface quality. Another type of conductive enclosure is made of a plastic material that is coated with a conductive layer. Although such coated-plastic enclosures tend to be relatively lightweight, the processes used for forming the conductive layer on the plastic material are expensive, since multiple coating steps are often required to deposit a continuous layer having the desired surface resistivity.
Telecommunication equipment is specified by industry standard documents. Telecommunication network switch equipment has the requirement of providing gaps of at least 1/8 of an inch between adjacent faceplates on a shelf. The gaps prevent potential binding between the faceplates. However, the gaps produce slots for EMI emission. One conventional solution uses metal gaskets positioned within the gaps of adjacent faceplates to provide EMI shielding.
It is desirable to provide a coextruded faceplate and gasket providing ESD immunity and EMI shielding of the network switch.